    Patent Document 1: JP-A-2003-130859    Patent Document 2: Japanese Patent No. 3704065    Nonpatent Literature 1: Takeko Murakami, Dominique Braconnier, Shunji Miura, Junichi Murai, Yutaka Nishitani, Proceedings of the 13th Symposium on Ultrasonic Testing, pp. 33-38, (2006)    Nonpatent Literature 2: Yoshikazu Yokono, Present Situation of Standardization of Phased Array UT, NDI Document 21776, pp. 34-38 (2006)    Nonpatent Literature 3: Masashi Mizuno, Hisao Nakase, Hiro Koda, Non-Destructive Inspection (Journal of the Japanese Society for Non-Destructive Inspection), 37(11), pp. 861-868, (1988)    Nonpatent Literature 4: Ultrasonic Flaw Detection of Steel, edited by the Iron and Steel Institute of Japan, Joint Research Group, Committee of Non-Destructive Inspection under Quality Control Society, pp. 79, (1993)
In a conventional flaw detection method using a single transducer, when internal flaw detection of a rod having the shape of a rectangular column, for example, is performed, it is necessary to make the transducer perform scanning mechanically not only along an axial direction of the rod but also, with respect to a cross section thereof intersecting with the axial direction, along the sides of a rectangular shape of a cross section of a material being tested.
In ultrasonic flaw detection of a material being tested, the ultrasonic flaw detection which has recently come into widespread use and is performed by using a phased array probe (hereinafter referred to as an array probe), a direction in which ultrasonic waves propagate and a point (focus) to which the ultrasonic waves converge can be arbitrarily set without a change in the placement of transducers by changing timing with which the individual transducers emit the ultrasonic waves (by performing phase control) (Patent Document 1).
As a result, in the above-described ultrasonic flaw detection using a phased array probe, instead of performing scanning mechanically along the sides of the material being tested, scanning is performed electrically.
This scanning is performed as follows. Instead of moving each transducer itself physically, as shown in FIG. 4 of Patent Document 1, the arranged transducers are sequentially vibrated in a time-shared manner so that a unit of a predetermined number of transducers is vibrated at a time. That is, of the transducers arranged in a scanning direction, a predetermined group of consecutive transducers emits ultrasonic waves, then a shift in the scanning direction is performed, and the next group emits the ultrasonic waves. By performing such a shift, it is possible to obtain the same effects as those obtained when the transducers are made to scan physically.
In addition, as a flaw detection technique that enables faster and higher-resolution flaw detection with higher detection capability based on the above-described conventional phased array flaw detection technique, a volume focusing phased array (hereinafter referred to as volume focusing if necessary) has been proposed (Nonpatent Literature 1 and Patent Document 2).
The above-described phased array flaw detection method has made remarkable advance in the past decade or so, and many apparatuses including a portable flaw detector and automatic flaw detection equipment have come to be used. This is attributed to the following reasons: a high-performance and low-cost flaw detector is made possible by the advancement of semiconductor technology and computer technology, and high-performance array search units (array probes) which are uniform in quality can be produced by the advent of a composite transducer.
The phased array flaw detection method finds wide application in ISI (In Service Inspection) of a nuclear power plant, an inspection of the fuselage and wings of an aircraft, online equipment used in the steel industry, and the like. Moreover, the movement toward standardization has also become active (Nonpatent Literature 2). In Japan, a phased array method is used also in an ultrasonic certification system in PD (Performance Demonstration) and achieves satisfactory results.
Volume focusing is a technique that enables faster and higher-resolution flaw detection with higher detection capability in these applications.
Hereinafter, applications of volume focusing will be described based on the principles thereof.
As volume focusing ultrasonic flaw detection equipment, desktop equipment and online-capable equipment have been proposed.
The desktop equipment is suitable for use in a field or for research purposes, has a flaw detection data analysis capability, and is adaptable to a matrix probe, which will be described later.
The online-capable equipment has a capability required for online automatic flaw detection, has a high-speed judgment capability, and can use a plurality of probes by performing parallel running.
Here, prior to an explanation of volume focusing, the above-described phased array flaw detection technique will be explained in more detail.
The basics of a conventional phased array inspection technique is to set a delay pattern to a virtual probe so that a group of transducers (a group of transducers performing transmission and reception concurrently: a virtual probe) obtains the same result as a focusing lens. An electric circuit of an array flaw detector scans each transmitted pulse (called a cycle or a time slot) at high speed at different settings. This operation may be considered as performing flaw detection by making the virtual probes that are set differently scan in sequence. Therefore, such array flaw detection has a great advantage over flaw detection using a single probe.
However, as is the case with multimode flaw detection, a temporal restriction is put on this method because transmission and reception is performed on a cycle-by-cycle basis. When a PRF (pulse repeat frequency) increases, a ghost echo caused by a multiple echo due to the front surface or a multiple echo in the material occurs, affecting a flaw detection speed. This problem is similar to that of a single probe.
That is, since this method repeats the following process: transmit and receive the ultrasonic waves, then perform electronic scanning, and then transmit and receive the ultrasonic waves again, the next transmission and reception of the ultrasonic waves cannot be performed until a ghost echo caused by the former emission of the ultrasonic waves is attenuated and ceases to exert an influence. This makes it necessary to lengthen a cycle of the former transmission and reception of the ultrasonic waves and the next transmission and reception of the ultrasonic waves.
On the other hand, as a method that enables high-sensitive flaw detection with high azimuth resolution, there is a zone focusing technique. The zone focusing technique performs flaw detection by connecting the focuses with respect to a zone set in a depth direction by performing transmission and reception while performing linear scanning. The focus can be set hierarchically, and high-sensitive flaw detection with high azimuth resolution is made possible by matching the focus in transmission and reception. Moreover, dynamic depth focusing (hereinafter referred to as DDF) can obtain a plurality of reception focuses with respect to one transmission, which is similar to having focuses with different depths with respect to one virtual probe, and is therefore effective in achieving a speedup.
However, in either method, there is a limit to a speedup since the method performs (electronic) scanning while making each virtual probe transmit and receive ultrasonic waves. Furthermore, since a large aperture cannot be formed in the current virtual probe having about 16 to 32 channels, making it impossible to lengthen the focal length. Thus, there is a limit to flaw detection of a thick and large object.
Unlike the above-described conventional phased array, volume focusing performs transmission with all the elements of an array probe at a time, then performs reception with all the elements, combines A scope waveforms of the elements, the A scope waveforms stored in a memory, and performs evaluation.
In the case of a linear probe, a transmitted wave propagates as a plane wave because it is emitted from a probe having a wide aperture. A reflection echo is amplified by an amplifier connected to all the elements and subjected to analog-to-digital conversion, and is then stored in the memory. In other words, the A scope waveforms of all the elements (for example, 128 elements) are stored in the memory in one transmission. This flaw detection waveform data is subjected to reception delay processing such as DDF on a set aperture-by-set aperture basis by signal processing performed by a high-speed DSP (Digital Signal Processor), and is evaluated. The above processing is performed at high speed, and more than one processing operation is performed simultaneously, whereby it is possible to achieve a further increase in the processing speed. When all the processes are finished, it becomes possible to perform the next transmission, and, if a ghost echo disappears during this time, the transmission can be performed. That is, it is possible to evaluate all the points in one cross section in one transmission without being affected by a ghost.
This is the reason why volume focusing is suitable for high-speed flaw detection.
For example, when internal flaw detection of a cross section of a rod-shaped material being tested is performed by disposing an array probe along the outer perimeter of the material being tested, the probe is made to perform scanning mechanically in an axial direction of the material being tested after the flaw detection of the cross section, whereby flaw detection of a cross section at another position in the axial direction is performed, and volume focusing is used in the above-described flaw detection of each cross section, it is possible to achieve a great reduction in flaw detection time at each position in the axial direction. This makes it possible to achieve a substantial reduction in the whole flaw detection time required for one rod.
In FIG. 10, a time chart of signal processing of volume focusing is shown.
T1 in FIG. 10 represents a transmitted wave of first ultrasonic waves, and T2 in FIG. 10 represents a transmitted wave of second ultrasonic waves. In both the first and second ultrasonic waves, S1 is a reflection echo reflected from the front surface of a material being tested, B1 is a reflection echo reflected from the bottom surface of the material being tested, and S2 is a reflection echo generated as a result of B1 being reflected again from the front surface of the material being tested. S2 to Sn are echoes called the ghost echoes described above.
By using FIGS. 11(A) and (B), a difference between zone focusing flaw detection and volume focusing flaw detection will be explained, taking up as an example flaw detection using a linear array probe having 128 elements.
Here, as conventional zone focusing flaw detection, a case where an array probe having 128 transducers is used and three strata are provided in a depth direction by performing simultaneous excitation of 32 elements is considered.
Specifically, each of the grids shown in an upper portion of FIG. 11(A) represents each of the elements of an array probe. An element represented by the grid on the extreme left is referred to as a first element, an element located next to the first element on the right is referred to as a second element, and an element next to the second element on the right is referred to as a third element. In this case, an element on the extreme right is a 128th transducer. Each element performs transmission and reception.
For each stratum to be subjected to flaw detection, first transmission and reception of the ultrasonic waves is performed by vibrating the first to 32nd elements, second transmission and reception of the ultrasonic waves is then performed by vibrating the second to 33rd elements, and third transmission and reception of the ultrasonic waves is then performed by vibrating the third to 34th elements. In this way, a group of 32 elements which perform emission simultaneously is shifted to the right, and the 126th to 128th elements are finally vibrated, whereby a total of 97 transmission and reception operations are performed. Such operation is electronic scanning by the array probe.
In the above-described flaw detection, signals for exciting the 32 elements forming one transmission and reception group are delayed differently. Moreover, signals obtained by the vibration due to the reception performed by these 32 elements are also delayed individually. As a result of such delay processing performed on transmission and reception, the ultrasonic waves emitted from the 32 elements at a time are focused on one point.
Then, by setting the focus of the array to a position z-1 serving as a first stratum with respect to a depth direction of a material being tested, the above-described electronic scanning is performed in a direction of arrow in FIG. 11(A) (the depth direction of the material being tested is a vertical direction in FIG. 11(A), and the direction of arrow is a horizontal direction of the drawing as shown in the drawing). When the flaw detection at each position in the direction of arrow is finished in the above-described first stratum, the focus of the array is then set to a position z-2 serving as a second stratum which is deeper than the first stratum, and electronic scanning is performed in the direction of arrow in the same manner as described above. When the flaw detection in the second stratum is finished, the focus of the array is then set to a position z-3 serving as a third stratum which is deeper than the second stratum, and electronic scanning is performed in the direction of arrow in the same manner as described above.
As described above, in this example, in the zone focusing flaw detection, three electronic scanning operations are required.
Therefore, in this example, it is necessary to perform 97 scanning operations in the element direction and three scanning operations in the depth direction, and actual ultrasonic wave transmission and reception is performed 97×3=291 times.
On the other hand, in the volume focusing flaw detection, it is possible to perform flaw detection by performing DDF on the above-described three strata or more than three strata in one transmission and reception. For example, in FIG. 11(B), volume focusing processing by which DDF is performed on five strata is shown, and an increase in the number of strata subjected to DDF does not affect the PRF.
A specific explanation is described below.
In FIG. 11(B), a plurality of parallel vertical lines extending downward from the array probe represent plane waves of simultaneous excitation of all the channels, and dashed lines represent a focus beam at the time of reception. A black circle represents the focus on the receiving side. That is, in the volume focusing flaw detection, the above-described 128 elements emit ultrasonic waves simultaneously, and the focus is not obtained at the time of transmission, and the focus is virtually obtained by delay processing at the time of reception.
As shown by the above-described vertical lines in FIG. 11(B), by making all the elements emit ultrasonic waves simultaneously at a time as described above, an echo received by each element is delayed, whereby the focus is virtually created. As a result, for example, for the ultrasonic waves received by the first to 32nd elements, reception processing by which the six black circles on the extreme left in FIG. 11(B) are each set as the focus can be performed at a time, and, by the next reception processing, reception processing by which the six black circles next to the above six black circles on the right are each set as the focus can be performed at a time. Such reception processing is performed 97 times, whereby processing in each stratum in the depth direction can be finished.
As described above, in the volume focusing flaw detection shown in FIG. 11(B), unlike the zone focusing flaw detection shown in FIG. 11(A), there is no need for electronic scanning, and the result of focusing on each position in the depth direction can be obtained. Thus, by transmitting and receiving ultrasonic waves once, it is possible to perform flaw detection in a range that would require a plurality of electronic scanning operations in the zone focusing flaw detection.
When a rod-shaped material being tested is taken as an example, T2 in FIG. 10 described above represents a transmitted wave for flaw detection of the next cross section, the cross section located in a position different from the cross section subjected to flaw detection by emission of T1 with respect to the axial direction of the material being tested. On the other hand, in the zone focusing flaw detection of FIG. 11(A), T1 is, for example, a transmitted wave emitted for obtaining the first focus in the first strata, and T2 is a transmitted wave emitted for obtaining the next focus located next to the above focus in the first strata with respect to the electronic scanning direction.
In both the zone focusing flaw detection and the dynamic focusing flaw detection, between S1 and B1 (which in actuality is a position located rather on the right side of B1 and near B2) of FIG. 10, the presence or absence of a defective echo is checked. In volume focusing, between S1 and B1, A scope capture processing is performed (a peak waveform such as B2 appearing on the right side of B1 is unnecessary because it is generated by a ghost echo, and therefore is not captured).
However, in zone focusing, since the next T2 is transmitted for the same cross section of the material being tested as the cross section to which T1 is transmitted, transmission of T2 cannot be performed until a ghost echo of T1 disappears.
The inventors tested how fast processing of the volume focusing flaw detection was as compared with zone focusing by using an aluminum test piece as a square billet. An artificial defect provided in this test piece is an SDH (Side Drill Hole) with a diameter of 0.5 mm. In both zone focusing and volume focusing, an array probe with 0.5 mm pitches and operating at 10 MHz was used. In the zone focusing method, scanning is performed in a depth direction in three stages of focal depth at 15-mm intervals and at a pitch of 0.5 mm in a longitudinal direction. To avoid a ghost, a PRF in each cycle was 2 KHz, and, overall, the PRF was 2000÷97÷3=6.8 Hz. On the other hand, in volume focusing, 128 elements were excited simultaneously, reception was performed by a focal row of 32 elements, 10-mm DDF was performed in a depth direction, and 0.5-mm pitch signal processing was performed. At this time, the PRF wave was 437 Hz. This is 64-times speedup in flaw detection.
Moreover, zone focusing was confirmed to have an excellent resolution capability because zone focusing could narrow focus in both transmission and reception. On the other hand, volume focusing was confirmed to have the focus because the beam was not spread in the depth direction due to the effect of the DDF. In volume focusing, B scope of the flaw detection can be obtained by one transmission.
As described above, as compared with the previous flaw detection method such as zone focusing, the above-described volume focusing has a great advantage in reducing the flaw detection speed. However, in particular, when internal flaw detection of a metal rod in the shape of a rectangular column, the metal rod called a square billet and having a rectangular cross-sectional shape, is performed, the presence of a dead zone in which adequate flaw detection cannot be performed becomes a problem (for conventional flaw detection of a square billet, see Nonpatent Literatures 3 and 4).
This problem will be described in detail.
Flaw detection of the entire internal area of a square billet may seem to be completed by one transmission and reception by performing, as flaw detection using the above-described volume focusing, a so-called vertical flaw detection method by which one array probe is disposed such that a plurality of transducers are arranged along one side of a square of a cross section of the square billet, as seen in its sectional view, almost from end to end of the width of that side, and pseudo plane waves generated by making the transducers emit ultrasonic waves simultaneously are made to enter perpendicularly that side as an incident side.
However, in reality, due to a reflection echo (a front surface echo) generated on the above-described incident side (an upper side), a region near the incident side in the square billet is a dead zone (a dead band) in which it is difficult to detect a defective echo.
Furthermore, due to a reflection echo (a bottom surface echo) generated on a counter side (a bottom side) facing the incident side, although the size is much smaller than that of the dead zone appearing on the incident side, a small dead zone also appears near the counter side in the square billet.
Moreover, as is apparent from the right and left ends of FIG. 11(B) described above, a region in which focus cannot be achieved by reception processing exists also near two adjacent sides adjacent to the incident side, the two adjacent sides of the square billet having a rectangular cross-sectional shape.